1. Field of the Invention
The present invention concerns a method and an apparatus for representing an x-ray image of an examination subject.
2. Description of the Prior Art
In the display rendering of x-ray images as they are acquired, for example, with a mobile C-arm x-ray apparatus, it is intended that image regions that are diagnostically relevant for the physician be shown with optimal quality on a monitor or in a presentation as a hard copy (film). This diagnostically relevant image region is formed by the region of the image field in which the image of the subject (the patient) is located. This region of the image field (designated in the following as the subject image region) is surrounded by regions that normally contain no diagnostic information. These are diaphragm-blanked regions as well as direct radiation regions in which x-rays are detected that have not penetrated the examination subject and thus are unattenuated.
Optimal quality means that the grey scale values within the subject image region enable an optimally differentiated, high-contrast representation of structures within this subject image region, with the representation being easily recognized and able for diagnostically evaluation. In contrast, the remaining image regions should be represented such that they do not hinder an observer. A primary problem in the representation of x-ray images is the direct radiation regions that occur with significant brightness in the x-ray image.
The image data at the output of an x-ray receiver (detector) that have already undergone pre-processing, so that the measured intensity I of the x-ray radiation is provided as a function of the image coordinates (x, y), normally exist with a resolution (for example 4096 intensity levels) that cannot be used by the representation medium (for example a monitor with 256 grey levels).
In order to achieve an image representation that is as optimal as possible, these intensity values must be mapped to the grey scale values that can be shown in the representation medium with apparatus-specific (normally non-linear) transformation rules (lookup characteristic curves or lookup tables). In other words, the grey scale values used for the image representation are associated with the respective intensities of the individual image points. For the determination of these grey scale values, the overall x-ray image is always evaluated in known x-ray apparatuses, because the presence or the position of direct radiation regions in the x-ray image is not known in advance. This has the consequence that the grey scale values in the subject image region are no longer shown with the best possible contrast resolution of the monitor, i.e. no longer utilizing the maximum possible grey scale values or grey level (gradation) range with, for example, 256 grey values, but instead are shown with fewer grey levels since, in the transformation, the extreme values (diaphragm region and direct radiation region) of the intensities measured by the x-ray receiver significantly limit the remaining grey scale value range available for the subject image region in the transformation.
Moreover, direct radiation regions that are present in the x-ray image can significantly mislead or distract or deceive the observer. The contrast resolution capability of the eye and the fine contrast detail in the subject image region are reduced when such direct radiation regions are reproduced on the monitor but are not recognized as such. Moreover, a darkening is always objectionable and should already be largely avoided for this reason.
The possibility exists in principle to minimize the direct radiation regions by the use of diaphragms (X-iris diaphragms or filter diaphragms). In practice, however, this possibility is not utilized because a correct adjustment of the diaphragm (in particular in the acquisition of a number of x-ray exposures from different directions) is time-consuming and moreover, adjustment of a diaphragm in a manner that prevents direct radiation regions without loss of diagnostic information is not possible in all cases due to a complex geometric shapes of the examination subject.